1. Field of the Invention
The present invention relates to a location (position) system, and more particularly, to a system and method for determining location (position) configured to minimize a measurement error of tag location.
2. Description of the Related Art
In 2.4 GHz real time locating system (RTLS) according to ISO/IEC 24730-2 international standard, a tag transmits a tag signal at a predetermined period, and a time difference of arrival (TDOA) by the tag signal to a multiple reader is calculated to estimate a tag location. To this end, at least three readers must receive signals transmitted by a tag.
FIG. 1 illustrates a tag signal transmitted by a tag.
Referring to FIG. 1, a tag repeatedly transmits a blank signal, DSSS (Direct Sequence Spread Spectrum) including a tag ID (Identification), or a sub-blank signal including same data. The blank signal or the sub-blank signal, although not illustrated in FIG. 1, is a signal where a tag ID or tag data information and PN (Pseudo Noise) code are EXOR (Exclusive-OR) performed, and transformed to the DSSS.
FIG. 2 illustrates a PN code generator generating a PN code. Referring to FIG. 2, the PN code generator generates a PN code having 511 chips using PN spread code Ox1CB.
According to the ISO/IEC 24730-2 international standard, a tag must transmit a blank signal having ±638 ms (milliseconds) interval within a period, i.e., within a time section of minimum 5 seconds or more, or transmit maximum eight sub-blank signals at 125 ms±16 ms interval within the same time section. At this time, it should be apparent that synchronization must be realized among the readers.
The reader having received the tag signal transmits the tag signal to a location determining engine, where the location determining engine calculates a tag location using a time difference of arrival (TDOA).
FIG. 3 illustrates a method for estimating a distance to a tag using the TDOA. Referring to FIG. 3, the TDOA read by a pair of readers may draw a pair of hyperbolas (dTOA23, dTOA12) having a predetermined difference of distance with the reader as vertex. An intersection point (tag) of these hyperbolas is estimated as a tag location.
A wireless communication environment creates problems attenuation, reflection, scattering and refraction of radio signal due to atmospheric temperature, humidity change and numerous communication obstacles. The problems may delay a tag transmission signal that must be measured, or attenuate the signal itself to disable measurement of arrival time of signal or increase an error thereof.